1. Field of the Invention
This invention relates to an electropneumatic converter with solenoid valve control which by imposing a constant minimum pressure difference across each of the two control solenoid valves not only results in an evident considerable reduction in the energy required for operating said solenoid valves but also results in a substantially linear variation in converter output pressure with solenoid valve opening time, independently of feed pressure variations and the value of the converter output pressure, besides maintaining the dead times of the solenoid valves always constant with consequent improvement in control precision and repeatability, in particular for small solenoid valve opening times.
2. Discussion of Background
As is well known, the pneumatic actuators of process valves are controlled by electropneumatic converters able to modulate their output pressure on the basis of electrical command signals. Various types of electropneumatic converters are already known in the state of the art.
In one of the known types, namely the type controlled by solenoid valves, a loading solenoid valve connected to the feed pressure and an unloading solenoid valve connected to atmosphere regulate the pressure within a capacitive chamber connected to them in series. The capacitive chamber pressure is then used to control a booster unit which is connected to the feed pressure, its purpose being to amplify the output flow rate from the converter to the extent necessary for controlling the connected actuators. Finally, to prevent the large movements of the moving element of the booster being able to vary the volume of the capacitive chamber with consequent undesirable variation in the pressure of this latter, a pressure repeater comprising a separation membrane, a restriction and a nozzle-plate system is interposed between the chamber and the booster.
Such a known converter has however the serious drawback that the output pressure signal does not vary linearly with the actuation or opening time of the solenoid valves in that it depends strongly on both the feed pressure and the value of the output pressure. This dependency means that the solenoid valves have to operate several times in order, by successive approximations, to achieve the required output pressure, with consequent considerable energy consumption. The drawbacks substantially derive from the fact that the pressure difference across the two solenoid valves varies depending on the converter operating conditions.
This variation in the pressure difference results in the further drawbacks that a fairly large operating force for the solenoid valves has to be available, with the consequent need for high electrical powers in play, and that the dead time of the solenoid valves, ie the time interval between the energization and the response of the solenoid valves, also varies, with consequent behaviour non-repeatability and poor operating precision of the solenoid valves, in particular for small opening times thereof.